Harmonic suppression in bulk acoustic wave duplexer

ABSTRACT

Harmonic suppression in bulk acoustic wave duplexer. In some embodiments, a filter circuit can include an input node and an output node, and a first assembly having one or more bulk acoustic wave (BAW) resonators implemented electrically between the input node and the output node, and configured to filter a signal. The filter circuit can further include a second assembly having one or more surface acoustic wave (SAW) resonators implemented electrically relative to the first assembly, and configured to suppress one or more harmonics resulting from the filtering of the signal by the first assembly.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims priority to U.S. Provisional Application No. 62/897,953 filed Sep. 9, 2019, entitled HARMONIC SUPPRESSION IN BULK ACOUSTIC WAVE DUPLEXER, the disclosure of which is hereby expressly incorporated by reference herein in its respective entirety.

BACKGROUND Field

The present disclosure relates to bulk acoustic wave (BAW) devices such as duplexers.

Description of the Related Art

In radio-frequency applications, bulk acoustic wave (BAW) devices can be utilized as filter devices. In some embodiments, such BAW filters can be implemented in duplexers.

SUMMARY

In accordance with some implementations, the present disclosure relates to a filter circuit that includes an input node and an output node, and a first assembly having one or more bulk acoustic wave (BAW) resonators implemented electrically between the input node and the output node, and configured to filter a signal. The filter circuit further includes a second assembly having one or more surface acoustic wave (SAW) resonators implemented electrically relative to the first assembly, and configured to suppress one or more harmonics resulting from the filtering of the signal by the first assembly.

In some embodiments, the first assembly can be configured as a band-pass filter with a frequency band about a center frequency f₀. The second assembly can be configured to suppress a second harmonic having a frequency of 2f₀. The second assembly can be configured as a notch filter with a frequency band about the second harmonic frequency 2f₀.

In some embodiments, the second assembly can be implemented electrically between the first assembly and the output node. The first assembly can include a series path having a plurality of BAW resonators electrically arranged in series between the input node and the second assembly. The first assembly can further include one or more BAW resonators each electrically arranged as a shunt path between a respective node along the series path and ground.

In some embodiments, the first assembly and the second assembly can be configured to support a transmit operation, and the signal can be a power-amplified signal. The output node can be an antenna node. The filter circuit can further include a third assembly having one or more bulk acoustic wave (BAW) resonators implemented electrically between the antenna node and a receive node, and be configured to filter a received signal from the antenna node. The first assembly and the third assembly can be configured to provide duplexing functionality.

In a number of implementations, the present disclosure relates to a duplexer that includes an antenna node, a transmit node, and a receive node. The duplexer further includes a transmit filter implemented to be electrically between the transmit node and the antenna node, and configured to filter a power-amplified signal. The transmit filter includes one or more bulk acoustic wave (BAW) resonators. The duplexer further includes a receive filter implemented to be electrically between the antenna node and the receive node, and configured to filter a received signal. The receive filter includes one or more BAW resonators, with at least one of the transmit filter and the receive filter further including a surface acoustic wave (SAW) resonator implemented electrically relative to the one or more BAW resonators of the respective filter, and configured to suppress one or more harmonics resulting from the filtering of the respective signal.

In some embodiments, the SAW resonator can be implemented to be electrically between the one or more BAW resonators of the transmit filter and the antenna node to suppress the one or more harmonics resulting from the filtering of the power-amplified signal. The one or more harmonics resulting from the filtering of the power-amplified signal can include a second harmonic.

In some teachings, the present disclosure relates to a filter device that includes a substrate and a filter circuit implemented on the substrate. The filter circuit includes an input node and an output node. The filter circuit further includes a first assembly having one or more bulk acoustic wave (BAW) resonators implemented electrically between the input node and the output node, and configured to filter a signal. The filter circuit further includes a second assembly having one or more surface acoustic wave (SAW) resonators implemented electrically relative to the first assembly, and configured to suppress one or more harmonics resulting from the filtering of the signal by the first assembly.

In some embodiments, each BAW resonator can include a piezoelectric material layer implemented between a lower electrode and an upper electrode. In some embodiments, the lower electrode can be on an upper surface of the substrate.

In some embodiments, each SAW resonator can include a piezoelectric material layer implemented over the substrate, and an interdigital transducer structure implemented over the piezoelectric material layer. In some embodiments, the piezoelectric material layer of the SAW resonator and the piezoelectric material layer of the BAW resonator can be formed from substantially the same material. In some embodiments, the piezoelectric material layer of the SAW resonator can be implemented directly on an upper surface of the substrate. In some embodiments, the piezoelectric material layer of the SAW resonator can be implemented on a metal layer which is on an upper surface of the substrate. In some embodiments, each of the piezoelectric material layer of the SAW resonator and the piezoelectric material layer of the BAW resonator can include aluminum nitride.

In some embodiments, the first assembly and the second assembly can be configured to support a transmit operation, and the signal is a power-amplified signal. The output node can be an antenna node. In some embodiments, the filter device can further include a third assembly having one or more bulk acoustic wave (BAW) resonators implemented electrically between the antenna node and a receive node, and configured to filter a received signal from the antenna node. The first assembly and the third assembly can be configured to provide duplexing functionality.

In some implementations, the present disclosure relates to a radio-frequency module that includes a packaging substrate configured to receive and support a plurality of components, and a die mounted on the packaging substrate and including a radio-frequency integrated circuit. The radio-frequency module further includes a filter device mounted on the packaging substrate and configured to support operation of the radio-frequency integrated circuit. The filter device includes a filter substrate and a filter circuit implemented on the filter substrate. The filter device further includes a first assembly having one or more bulk acoustic wave (BAW) resonators implemented electrically between an input node and an output node, and configured to filter a signal. The filter device further includes a second assembly having one or more surface acoustic wave (SAW) resonators implemented electrically relative to the first assembly, and configured to suppress one or more harmonics resulting from the filtering of the signal by the first assembly.

According to some implementations, the present disclosure relates to a wireless device that includes an antenna and a front-end system in communication with the antenna. The front-end system includes a filter circuit having an input node and an output node. The filter circuit further includes a first assembly having one or more bulk acoustic wave (BAW) resonators implemented electrically between the input node and the output node, and configured to filter a signal. The filter circuit further includes a second assembly having one or more surface acoustic wave (SAW) resonators implemented electrically relative to the first assembly, and configured to suppress one or more harmonics resulting from the filtering of the signal by the first assembly.

For purposes of summarizing the disclosure, certain aspects, advantages and novel features of the inventions have been described herein. It is to be understood that not necessarily all such advantages may be achieved in accordance with any particular embodiment of the invention. Thus, the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other advantages as may be taught or suggested herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a side view of a bulk acoustic wave (BAW) resonator implemented on a substrate.

FIG. 2 shows an example of a filter that can be implemented using a plurality of BAW resonators such as the BAW resonator of FIG. 1 .

FIG. 3 depicts an amplitude distribution that can arise from use of the filter of FIG. 2 .

FIG. 4 shows that in some embodiments, a filter can include first and second nodes, and an assembly of one or more BAW resonators implemented between the first and second nodes.

FIG. 5 depicts an amplitude distribution that can arise from use of the filter of FIG. 4 .

FIG. 6 shows a more specific example of the filter of FIG. 4 .

FIG. 7 shows that in some embodiments, a combination of BAW and SAW assemblies can be implemented in a filtering device such as a duplexer.

FIG. 8 shows an example of a filter implemented on a common substrate.

FIG. 9 shows another example of a filter implemented on a common substrate.

FIG. 10 shows a perspective view of a SAW assembly that can be the SAW assembly in each of FIGS. 8 and 9 .

FIG. 11 shows that in some embodiments, a filter having a combination of BAW resonator(s) and SAW resonator(s) as described herein can be implemented as a filter device such as a filter die.

FIG. 12 shows that in some embodiments, a filter circuit having a combination of BAW and SAW resonators as described herein can be implemented as a duplexer device.

FIG. 13 shows that in some embodiments, a radio-frequency (RF) module can include a BAW/SAW assembly having one or more features as described herein.

FIG. 14 depicts an example wireless device having one or more advantageous features described herein.

DETAILED DESCRIPTION OF SOME EMBODIMENTS

The headings provided herein, if any, are for convenience only and do not necessarily affect the scope or meaning of the claimed invention.

Described herein are various examples related to bulk acoustic wave (BAW) filter devices such as duplexers. FIG. 1 depicts a side view of a BAW resonator 30 implemented on a substrate 12. Such a BAW resonator can be formed by positioning a piezoelectric material layer 20 between a first electrode (e.g., upper electrode 16) and a second electrode (e.g., lower electrode 14). If a radio-frequency (RF) signal is applied to the resonator 30 through one of the electrodes, a corresponding acoustic wave is propagated through the piezo layer 20 towards the other electrode and leaves that electrode as an RF signal corresponding to the frequency of the acoustic wave. Thus, the BAW resonator 30 can provide an excellent filtering functionality.

FIG. 2 shows an example of a filter 40 that can be implemented using a plurality of BAW resonators such as the BAW resonator 30 of FIG. 1 . In the example of FIG. 2 , the filter 40 is shown to include first and second nodes 42, 44. The first node 42 can be, for example, an input node for an RF signal, and the second node 44 can be an output node for the filtered RF signal.

The example filter 40 is shown to include three BAW resonators 30 a, 30 b, 30 c electrically arranged in series between the first and second nodes 42, 44. It will be understood that a filter can include more or less number of series resonators.

The example filter 40 is also shown to include two BAW resonators 30 d, 30 e each electrically arranged to provide a shunt coupling from a respective node along the series path to ground. More particularly, the BAW resonator 30 d is shown to provide a shunt coupling from a node between the series BAW resonators 30 a, 30 b to ground, and the BAW resonator 30 e is shown to provide a shunt coupling from a node between the series BAW resonators 30 b, 30 c to ground. It will be understood that a filter can include more or less number of shunt resonators.

It is noted that for a BAW resonator based filter, such as the example filter 40 of FIG. 2 , a significant amount of one or more harmonics can arise when the filter is provided with a signal having a frequency f₀. For example, suppose that the filter 40 of FIG. 2 is configured to provide a band-pass filter functionality with a center frequency of f₀. One or more harmonics can arise based on the center frequency of f₀(e.g., 2 f₀, 3 f₀, etc.); and if a harmonic is sufficiently large, it can cause problems in one or more circuits associated with the filter 40.

For example, FIG. 3 depicts an amplitude distribution that can arise from use of the filter 40 of FIG. 2 . As described above, suppose that the filter 40 of FIG. 2 is implemented with a center frequency of f₀. Accordingly, amplitude of a filtered signal at f₀ is depicted as having a given amplitude (“BAW” bar at f₀). FIG. 3 further shows that a second harmonic (2f₀) can have a relatively large amplitude (“BAW” bar at 2f₀).

In some embodiments, a BAW based filter having one or more features as described herein can include one or more resonators implemented to remove or reduce one or more harmonics associated with the BAW based filter. By way of an example, and as further shown in FIG. 3 , it is noted that a surface acoustic wave (SAW) filter having similar functionality as the BAW filter 40 of FIG. 2 has a relatively smaller amplitude harmonics when compared to those associated with the BAW filter 40. In FIG. 3 , the second harmonic (2f₀) associated with such a SAW filter is shown to have a relatively small amplitude (“SAW” bar at 2f₀) when compared to the BAW filter's second harmonic amplitude.

In some embodiments, the foregoing phenomenon of the SAW filter having smaller amplitude harmonic(s) may be due to the IDT (interdigital transducer) structure of the SAW filter. Thus, such a feature can be utilized to suppress one or more harmonics associated with a BAW filter.

For example, FIG. 4 shows that in some embodiments, a filter 100 can include first and second nodes 102, 104, and an assembly 110 of one or more BAW resonators implemented between the first and second nodes 102, 104. For the purpose of description, such an assembly of BAW resonators can be, for example, the series and shunt BAW resonators of FIG. 2 .

In some embodiments, an assembly 120 of one or more SAW resonators can be implemented on an output side of the BAW assembly 110. In such a configuration, a harmonic such as a second harmonic resulting from the filtering achieved in the BAW assembly 110 can be suppressed by the SAW assembly 120. Thus, in the example of FIG. 4 , if the first node 102 is an input node and the second node 104 is an output node, the SAW assembly 120 can be implemented on the output node side of the BAW assembly 110.

Configured in the foregoing manner, the filter 100 can have one or more of its harmonics suppressed. For example, FIG. 5 depicts an amplitude distribution that can arise from use of the filter 100 of FIG. 4 . Suppose that the filter 100 of FIG. 4 is implemented with a center frequency of f₀. Accordingly, amplitude of a filtered signal at f₀ is depicted as having a given amplitude 130. FIG. 5 further shows that a second harmonic (2f₀) can have a suppressed amplitude 132. Such a suppressed second harmonic amplitude can be significantly lower than a second harmonic amplitude level 134 of a filter (e.g., filter 40 of FIG. 2 ) without the SAW assembly 120.

FIG. 6 shows a more specific example of the filter 100 of FIG. 4 . In FIG. 6 , an example filter 100 is shown to have a BAW assembly 110 that is similar to the filter 40 of FIG. 2 . Implemented on one side of the BAW assembly 110 is a SAW assembly 120. Thus, assuming that node 102 is an input node, a signal to be filtered can pass through the BAW assembly 110 to, for example, be band-pass filtered by the BAW assembly 110 about a center frequency of f₀. The resulting harmonics such as a second harmonic (2f₀) present at the output of the BAW assembly 110 can then be suppressed by the SAW assembly 120.

In some embodiments, the SAW assembly 120 can include one or more SAW resonators electrically arranged in any manner to provide suppression of one or more harmonic frequencies associated with the center frequency of the BAW assembly. Such an arrangement of SAW resonators can include, for example, one or more SAW resonators electrically arranged in series, with or without one or more shunt (SAW) resonators.

In some embodiments, the SAW assembly 120 can be configured to provide a resonance frequency at or close to a harmonic frequency such as a second harmonic frequency 2f₀. Such a resonance frequency can be utilized to provide, for example, a notch filter functionality to suppress the second harmonic at the frequency 2f₀.

In the various examples described in reference to FIGS. 2-6 , a SAW assembly can be combined with a BAW assembly to provide a desired suppression of one or more harmonics in a filter configuration. FIG. 7 shows that in some embodiments, such a filter configuration having a combination of BAW and SAW assemblies can be implemented in a filtering device such as a duplexer 130.

In the example of FIG. 7 , the duplexer 130 is shown to include a BAW assembly 110 a implemented to receive a power-amplified signal at a node Tx for transmission through an antenna node ANT. A SAW assembly 120 is shown to be implemented between the BAW assembly 110 a and the antenna node ANT. Thus, the resulting combination 100 of the BAW assembly 110 a and the SAW assembly 120 can be similar to the example filter 100 of FIG. 6 . In the example of FIG. 7 , such a filter (100) is shown to be implemented as a transmit filter.

In the example of FIG. 7 , the duplexer 130 is shown to further include a BAW assembly 110 b implemented to receive a signal from the antenna node ANT and provide a receive filter functionality to provide a filtered receive signal to an output node Rx. In some embodiments, a SAW assembly similar to the transmit filter 100 may or may not be implemented with the BAW assembly 110 b.

In the example of FIG. 7 , the SAW assembly 120 is shown to be implemented on the output side of the transmit filter 100. Configured in such a manner, one or more harmonics (e.g., second harmonic) resulting from filtering of a power-amplified transmit signal by the BAW assembly 110 a can be suppressed by the SAW assembly 120 before reaching the antenna node ANT. If such a harmonic suppressor is not present on the output side of the transmit filter, relatively high amplitude harmonic(s) present at the antenna node ANT can interfere significantly with receive operations on the receive side of the duplexer 130, and/or transmit operations through the antenna node ANT.

FIGS. 8 and 9 show that in some embodiments, a filter 100 having one or more features as described herein can be implemented on a common substrate. For example, a filter 100 in each of FIGS. 8 and 9 can include a substrate 140, a BAW assembly 110 implemented on the substrate 140, and a SAW assembly 120 also implemented on the substrate 140. In each of FIGS. 8 and 9 , the BAW assembly 110 can include a piezoelectric material layer 146 between a first electrode (e.g., upper electrode 150) and a second electrode (e.g., lower electrode 142), with the lower electrode 142 being on the upper surface of the substrate 140. It will be understood that the BAW assembly 110 can have one or more of such resonator structures electrically coupled as described herein.

In each of FIGS. 8 and 9 , the respective SAW assembly 120 can include a piezoelectric material layer 148 and an interdigital transducer (IDT) structure 160 implemented over the piezoelectric material layer 148. In the example of FIG. 8 , such a SAW assembly is shown to be implemented over a metal layer 144. In some embodiments, such a metal layer can be, for example, a patterned portion of the same metal layer as the electrode 142. In some embodiments, the piezoelectric material layer 148 of the SAW assembly 120 can be formed from the same material as the piezoelectric material layer 146 of the BAW assembly 110. In some embodiments, such a piezoelectric material can include, for example, aluminum nitride (AIN). The thickness of the piezoelectric material layer 148 of the SAW assembly 120 may or may not be the same as the thickness of the piezoelectric material layer 146 of the BAW assembly 110. It will be understood that the SAW assembly 120 can have one or more of such SAW resonator structures electrically coupled to provide harmonic suppression functionality as described herein.

In the example of FIG. 9 , the SAW assembly 120 is shown to be implemented over the substrate 140 without an intervening metal layer. Referring to FIG. 9 , in some embodiments, the piezoelectric material layer 148 of the SAW assembly 120 can be formed from the same material as the piezoelectric material layer 146 of the BAW assembly 110. In some embodiments, such a piezoelectric material can include, for example, aluminum nitride (AIN). The thickness of the piezoelectric material layer 148 of the SAW assembly 120 may or may not be the same as the thickness of the piezoelectric material layer 146 of the BAW assembly 110. It will be understood that the SAW assembly 120 can have one or more of such SAW resonator structures electrically coupled to provide harmonic suppression functionality as described herein.

FIG. 10 shows a perspective view of a SAW assembly 120 that can be the SAW assembly in each of FIGS. 8 and 9 . More particularly, the SAW assembly 120 is shown to includes a piezoelectric material layer 148 such as an aluminum nitride (AIN) layer, and an interdigital transducer (IDT) structure 160 implemented over the piezoelectric material layer 148. Such an IDT structure can include a plurality of interdigitized metal traces 162 electrically connected respective electrodes. In some embodiments, the IDT structure 160 may or may not be implemented using the same metal as the upper electrode 150 of the respective BAW assembly 110.

For example, some or all of the IDT structure 160 can be formed using the same metal as the upper electrode 150 of the BAW assembly 110. In such an example, formation of the piezoelectric material layer 146 of the BAW assembly 110 and formation of the piezoelectric material layer 148 of the SAW assembly 120 can be achieved during the same deposition/pattern process. Following such a process, formation of the upper electrode 150 of the BAW assembly 110 and formation of the IDT structure 160 of the SAW assembly 120 can be achieved during the same deposition/pattern process.

FIG. 11 shows that in some embodiments, a filter 100 having a combination of BAW resonator(s) and SAW resonator(s) as described herein can be implemented as a filter device such as a filter die 200. Such a filter die can include a substrate 202 configured to support the filter 100.

FIG. 12 shows that in some embodiments, a filter circuit 130 having a combination of BAW and SAW resonators as described herein can be implemented as a duplexer device 210. Such a duplexer device can include a substrate 212 configured to support the filter circuit 130.

FIG. 13 shows that in some embodiments, a radio-frequency (RF) module 300 can include a BAW/SAW assembly 100 having one or more features as described herein. For the purpose of description of FIG. 13 , it will be understood that such a BAW/SAW assembly (100) can include the BAW/SAW filter circuit 130 of FIG. 12 implemented to provide a duplexing functionality.

In some embodiments, the RF module 300 of FIG. 13 can include a filter assembly 306, and one or more of the BAW/SAW assemblies 100 can be part of such a filter assembly (306). In some embodiments, the RF module 300 can also include, for example, an RF integrated circuit (RFIC) 304, and an antenna switch module (ASM) 308. Such an example module can be, for example, a front-end module configured to support wireless operations. In some embodiments, some of all of the foregoing components can be mounted on and supported by a packaging substrate 302.

In some implementations, a device and/or a circuit having one or more features described herein can be included in an RF device such as a wireless device. Such a device and/or a circuit can be implemented directly in the wireless device, in a modular form as described herein, or in some combination thereof. In some embodiments, such a wireless device can include, for example, a cellular phone, a smart-phone, a hand-held wireless device with or without phone functionality, a wireless tablet, etc.

FIG. 14 depicts an example wireless device 400 having one or more advantageous features described herein. In the context of a module having one or more features as described herein, such a module can be generally depicted by a dashed box 300, and can be implemented as, for example, a front-end module (FEM). In such an example, one or more BAW/SAW assemblies as described herein can be included in, for example, an assembly of filters such as duplexers 424.

Referring to FIG. 14 , power amplifiers (PAs) 420 can receive their respective RF signals from a transceiver 410 that can be configured and operated in known manners to generate RF signals to be amplified and transmitted, and to process received signals. The transceiver 410 is shown to interact with a baseband sub-system 408 that is configured to provide conversion between data and/or voice signals suitable for a user and RF signals suitable for the transceiver 410. The transceiver 410 can also be in communication with a power management component 406 that is configured to manage power for the operation of the wireless device 400. Such power management can also control operations of the baseband sub-system 408 and the module 300.

The baseband sub-system 408 is shown to be connected to a user interface 402 to facilitate various input and output of voice and/or data provided to and received from the user. The baseband sub-system 408 can also be connected to a memory 404 that is configured to store data and/or instructions to facilitate the operation of the wireless device, and/or to provide storage of information for the user.

In the example wireless device 400, outputs of the PAs 420 are shown to be routed to their respective duplexers 424. Such amplified and filtered signals can be routed to an antenna 416 through an antenna switch 414 for transmission. In some embodiments, the duplexers 424 can allow transmit and receive operations to be performed simultaneously using a common antenna (e.g., 416). In FIG. 14 , received signals are shown to be routed to “Rx” paths (not shown) that can include, for example, a low-noise amplifier (LNA).

Unless the context clearly requires otherwise, throughout the description and the claims, the words “comprise,” “comprising,” and the like are to be construed in an inclusive sense, as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to.” The word “coupled”, as generally used herein, refers to two or more elements that may be either directly connected, or connected by way of one or more intermediate elements. Additionally, the words “herein,” “above,” “below,” and words of similar import, when used in this application, shall refer to this application as a whole and not to any particular portions of this application. Where the context permits, words in the above Detailed Description using the singular or plural number may also include the plural or singular number respectively. The word “or” in reference to a list of two or more items, that word covers all of the following interpretations of the word: any of the items in the list, all of the items in the list, and any combination of the items in the list.

The above detailed description of embodiments of the invention is not intended to be exhaustive or to limit the invention to the precise form disclosed above. While specific embodiments of, and examples for, the invention are described above for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize. For example, while processes or blocks are presented in a given order, alternative embodiments may perform routines having steps, or employ systems having blocks, in a different order, and some processes or blocks may be deleted, moved, added, subdivided, combined, and/or modified. Each of these processes or blocks may be implemented in a variety of different ways. Also, while processes or blocks are at times shown as being performed in series, these processes or blocks may instead be performed in parallel, or may be performed at different times.

The teachings of the invention provided herein can be applied to other systems, not necessarily the system described above. The elements and acts of the various embodiments described above can be combined to provide further embodiments.

While some embodiments of the inventions have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the disclosure. Indeed, the novel methods and systems described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the methods and systems described herein may be made without departing from the spirit of the disclosure. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the disclosure. 

What is claimed is:
 1. A filter circuit comprising: an input node and an output node; a first assembly having one or more bulk acoustic wave (BAW) resonators implemented electrically between the input node and the output node, and configured to filter a signal as a band-pass filter with a frequency band about a center frequency f₀; and a second assembly having one or more surface acoustic wave (SAW) resonators implemented electrically relative to the first assembly, and configured to suppress a second harmonic having a frequency of 2f₀ resulting from the filtering of the signal by the first assembly.
 2. The filter circuit of claim 1 wherein the second assembly is configured as a notch filter with a frequency band about the second harmonic frequency 2f₀.
 3. The filter circuit of claim 1 wherein the second assembly is implemented electrically between the first assembly and the output node.
 4. The filter circuit of claim 3 wherein the first assembly includes a series path having a plurality of BAW resonators electrically arranged in series between the input node and the second assembly.
 5. The filter circuit of claim 4 wherein the first assembly further includes one or more BAW resonators each electrically arranged as a shunt path between a respective node along the series path and ground.
 6. The filter circuit of claim 3 wherein the first assembly and the second assembly are configured to support a transmit operation, and the signal is a power-amplified signal.
 7. The filter circuit of claim 6 wherein the output node is an antenna node.
 8. The filter circuit of claim 7 further comprising a third assembly having one or more bulk acoustic wave (BAW) resonators implemented electrically between the antenna node and a receive node, and configured to filter a received signal from the antenna node.
 9. A duplexer comprising: an antenna node, a transmit node, and a receive node; a transmit filter implemented to be electrically between the transmit node and the antenna node, and configured to filter a power-amplified signal, the transmit filter including one or more bulk acoustic wave (BAW) resonators; and a receive filter implemented to be electrically between the antenna node and the receive node, and configured to filter a received signal, the receive filter including one or more BAW resonators, at least one of the transmit filter and the receive filter further including a surface acoustic wave (SAW) resonator implemented electrically relative to the one or more BAW resonators of the respective filter, and configured to suppress one or more harmonics resulting from the filtering of the respective signal.
 10. The duplexer of claim 9 wherein the SAW resonator is implemented to be electrically between the one or more BAW resonators of the transmit filter and the antenna node to suppress the one or more harmonics resulting from the filtering of the power-amplified signal.
 11. The duplexer of claim 10 wherein the one or more harmonics resulting from the filtering of the power-amplified signal includes a second harmonic.
 12. A filter device comprising: a substrate; and a filter circuit implemented on the substrate, and including an input node and an output node, the filter circuit further including a first assembly having one or more bulk acoustic wave (BAW) resonators implemented electrically between the input node and the output node, and configured to filter a signal as a band-pass filter with a frequency band about a center frequency f₀, the filter circuit further including a second assembly having one or more surface acoustic wave (SAW) resonators implemented electrically relative to the first assembly, and configured to suppress a second harmonic having a frequency of 2f₀ resulting from the filtering of the signal by the first assembly.
 13. The filter device of claim 12 wherein each BAW resonator includes a piezoelectric material layer implemented between a lower electrode and an upper electrode.
 14. The filter device of claim 13 wherein each SAW resonator includes a piezoelectric material layer implemented over the substrate, and an interdigital transducer structure implemented over the piezoelectric material layer.
 15. The filter device of claim 14 wherein the piezoelectric material layer of the SAW resonator and the piezoelectric material layer of the BAW resonator are formed from substantially the same material.
 16. The filter device of claim 15 wherein the piezoelectric material layer of the SAW resonator is implemented directly on an upper surface of the substrate.
 17. The filter device of claim 15 wherein each of the piezoelectric material layer of the SAW resonator and the piezoelectric material layer of the BAW resonator includes aluminum nitride.
 18. A filter device comprising: a substrate; and a filter circuit implemented on the substrate, and including an input node and an output node, the filter circuit further including a first assembly having one or more bulk acoustic wave (BAW) resonators implemented electrically between the input node and the output node, and configured to filter a signal, the filter circuit further including a second assembly having one or more surface acoustic wave (SAW) resonators implemented electrically relative to the first assembly, and configured to suppress one or more harmonics resulting from the filtering of the signal by the first assembly, each BAW resonator including a piezoelectric material layer implemented between a lower electrode and an upper electrode, each SAW resonator including a piezoelectric material layer implemented over the substrate, and an interdigital transducer structure implemented over the piezoelectric material layer, the piezoelectric material layer of the SAW resonator and the piezoelectric material layer of the BAW resonator formed from substantially the same material, the piezoelectric material layer of the SAW resonator implemented on a metal layer which is on an upper surface of the substrate. 